The production of higher hydrocarbon materials from synthesis gas, i.e. carbon monoxide and hydrogen, commonly known as the Fischer-Tropsch (“F-T”) process, has been in commercial use for many years. Such processes rely on specialized catalysts. The original catalysts for the Fischer-Tropsch synthesis were nickel. Nickel is still the preferred catalyst for hydrogenation of fats and specialty chemicals. Over the years, other metals, particularly iron and cobalt, have become preferred in the Fischer-Tropsch synthesis of higher hydrocarbons whereas copper has been the catalyst of choice for alcohol synthesis. Cobalt is particularly preferred for Fischer-Tropsch synthesis due its high productivity and comparatively low methane selectivity. As the technology of these syntheses developed over the years, the catalysts have become more refined and have been augmented by other metals and/or metal oxides that function to promote their catalytic activity. Promoter metals or metal oxides include, without intended limitation, Ru, Os, Ir, Mo, W, Cu, Si, Cr, Ti, Mg, Mn, Zr, Hf, Al, Th and the like. It is generally recognized that the choice of a particular metal or alloy for fabricating a catalyst to be utilized in Fischer-Tropsch synthesis will depend in large measure on the desired product or products.
In 1924, M. Raney prepared a nickel hydrogenation catalyst by a process known today as the Raney Process. For purposes of simplicity, the term “Raney” will be utilized herein as a generic term to describe the process, alloys and catalysts obtained thereby. This specific synthesis, in essence, comprises forming at least a binary alloy of metals, at least one of which can be extracted, and extracting it thereby leaving a porous residue of the non-soluble metal or metals that possess catalytic activity. The residue, or non-extractable, catalyst metals are the art-recognized group of metals described above. The extractable metals, typically aluminum, are likewise an art-recognized group. Once alloys are formed of at least one member of each of these groups of metals, they are ground to a fine powder and treated with strong caustic, such as sodium hydroxide, to leach the extractable metal from the residue metal or metals.
There exist many variations of the basic preparation of Raney catalysts such as, for example, deposition of alloys onto a preformed support by flame spraying, (U.S. Pat. No. 4,089,812), formation of the alloy by surface diffusion of aluminum on a non-leachable metal substrate (U.S. Pat. No. 2,583,619), and forming pellets from the powdered alloys for use in fixed bed reactions vessels (U.S. Pat. No. 4,826,799, U.S. Pat. No. 4,895,994 and U.S. Pat. No. 5,536,694). These developments have made possible the use of shaped Raney catalysts in fixed bed reaction vessels.
Particularly suited for the production of hydrocarbons by Fischer-Tropsch synthesis from synthesis gas are Dispersed Active Metals (“DAM”) which are primarily, i.e. at least about 50 wt. %, preferably at least 80 wt. %, composed of one or a mixture of metals such as described above and are, without further treatment, capable of catalyzing Fischer-Tropsch synthesis. DAM catalysts may be prepared by any of a number of art-recognized processes. An extensive review of process of forming DAM catalysts can be found in “Active Metals”, Edited by Alois Furstner, published by VCH Verlagsgesellschaft mbH, D-69451 Weinheim (FRG) in 1996 and the references cited therein. Methodologies described therein include the Reike method, the use of ultrasound, reduction of metal salts, colloids, nanoscale cluster and powders. Other relevant references include, for example, the preparation of amorphous iron catalyst by high intensity sonolysis of iron pentacarbonyl, Suslick et al., Nature, Vol. 353, pp 414-416 (1991) and the formation of single domain cobalt clusters by reduction of a cobalt salt with hydrazine, Gibson et el., Science, Vol. 267, pp 1338-1340, (1998). Finally, intermetallic alloys, particularly those known for forming metal hydrides, such as LaCo5, can be formed into a fine powder by the application of hydrogen adsorption/desorption cycles. Such catalysts can also be prepared by thermal or chemical decomposition of metal formates or oxalates.
It will be appreciated that a means of producing DAM catalysts having enhanced activity for hydrogenation reactions would be of significant value. In particular, an important aspect of the value of a catalyst for the Fischer-Tropsch process, in addition to its activity, is its selectivity. This property, commonly referred to as “methane selectivity”, is the ratio of the percent of feed material converted to desired higher hydrocarbons to that of short chain hydrocarbons produced, primarily methane. In accordance with the present invention, it has been found that DAM catalysts possessing enhanced activity and methane selectivity can be produced from particulate metals, such as Fischer-Tropsch cobalt catalysts, by a process that is both simple and economical.